Researchers have found three species that apparently spend their entire lives in the oxygen-starved waters at the bottom of the Mediterranean Sea. Previously, living oxygen-free was thought to only be possible for viruses and single-celled microorganisms, but these three as-yet-unnamed bottom-dwellers -- measuring less than 1 millimeter long -- seem to exist without it. They live at a depth of more than 3,000 meters, in oxygen-free sediments at the bottom of the sea. It's not the first time that multicellular animals have been found in anoxic environments, but these are the first that seem to be permanent residents.